Removal of excess heat in a failed stirling converter in a radioisotope power system

ABSTRACT

A power supply system for converting heat to energy includes a heat source, a converter, a heat pipe, a heat transfer structure surrounding the heat pipe, and a mechanism for moving the heat pipe as a function of electrical power generated by the converter between a first position spaced from the converter and a second position adjacent to the converter. The converter is positioned adjacent the heat source and uses heat from the heat source. The heat pipe has a first end and a second end and is capable of withdrawing heat from the converter.

BACKGROUND OF THE INVENTION

The present invention generally relates to the field of radioisotope power systems. In particular, the invention relates to removing heat from a radioisotope power system.

Stirling Radioisotope Power Sources (RPS) typically use plutonium as a heat source, which can be used in combination with a Stirling engine to generate electricity. Other isotopes of radioactive materials may also be used as a heat source, including, but not limited to: strontium, cesium, cerium, promethium, and polonium. As the atoms of the plutonium decay, heat is produced. Because of the radioactive nature of plutonium, the plutonium is typically housed within a cladding and protective aeroshell to form a “brick”. For example, the plutonium may be housed in an iridium cladding, which provides a barrier to dispersion of the bricks in the case of a re-entry event of the plutonium. However, at high temperatures, the cladding becomes brittle and is less likely to contain the plutonium inside the brick. In order to maintain the temperature of the iridium cladding at acceptable operating temperatures, a radiator, power cycle, or heat path must be positioned adjacent to the radioactive brick to remove heat from the brick and ensure that the iridium cladding does not overheat. The typical method of removing heat is by a power cycle, such as a thermoelectric couple or Stirling engine.

In a Stirling radioisotope power system, between approximately 70% to approximately 80% of the heat that is produced from the plutonium brick is rejected. The remaining approximately 20% to 30% of the energy produced from the radioactive material is converted to electricity by the Stirling engine. One concern is that in the event that the converter fails, the thermal energy that is typically drawn from the brick to generate electricity in the converter will no longer be used to generate electricity, and heat will build up in the converter. Thus, the temperatures of the converter and the brick increase, leading to overheating of the bricks and potential embrittlement of the iridium cladding. In order to prevent overheating in the event of converter failure, an alternate heat path to remove the heat from the iridium is desired.

BRIEF SUMMARY OF THE INVENTION

A power supply system for converting heat to energy includes a heat source, a converter, a heat pipe, a heat transfer structure surrounding the heat pipe, and a mechanism for moving the heat pipe as a function of electrical power generated by the converter between a first position spaced from the converter and a second position adjacent to the converter. The converter is positioned adjacent the heat source and uses heat from the heat source. The heat pipe has a first end and a second end and is capable of withdrawing heat from the converter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is schematic of a power supply system having an operating converter.

FIG. 2 is a schematic of the power supply system of FIG. 1 having a failed converter.

FIG. 3 is a diagram of a method of removing heat from the power supply system.

DETAILED DESCRIPTION

FIG. 1 shows a schematic of power supply system 10 having an operating converter 12 for generating electricity. Power supply system 10 generally includes converter 12, heat source 14, power line 16, electromagnet 18, heat pipe 20, spring 22, and heat transfer structure 24. In one embodiment, power supply system 10 is a Stirling Radioisotope Power Supply (SRPS). Heat pipe 20 increases the safety of power supply system 10 by providing an alternate means of withdrawing heat from heat source 14 in the event that converter 12 is experiencing failure. Power supply system thus decreases the chance of radioisotope dispersion in the event of an accident. However, power supply system 10 also has an increased mass to power ratio compared to a similar system without a converter failure heat removal mechanism. This increases the overall mass of power supply system 10. However, even with the additional mass, the mass to power ratio of power supply system 10 is lower than other radioisotope power supply concepts, for example, Radioisotope Thermoelectric Generators (RTGs). The mass of power supply system 10 gives power supply system 10 the capability of being used on a flight unit, such as spacecraft.

Heat source 14 is a general purpose heat source (GPHS) that may be used to generate heat or electricity. Cladding 26 surrounds heat source 14 and prevents any radioactive material from escaping from heat source 14. In one embodiment, heat source 14 uses plutonium radioisotope pellets and is cladded with iridium cladding. The iridium cladding prevents the plutonium from escaping into the atmosphere. A shell 27 then completely houses heat source 14 and cladding 26. In an exemplary embodiment, shell 27 is an aeroshell. Although FIG. 1 is discussed as using plutonium as heat source 14, heat source 14 may use any radioactive material, including, but not limited to: strontium, cesium, cerium, promethium, and polonium.

Converter 12 is positioned adjacent to heat source 14 and uses a portion of the heat from heat source 14 to generate electricity. Because converter 12 is between approximately 20% to approximately 40% efficient, only a portion of the heat generated by heat source 14 is converted from heat into electricity. Converter 12 is typically formed of metal and is capable of withdrawing heat from heat source 14 through shell 27 or being a small distance from heat source 14 and heating by radiative heat transfer. In one embodiment, converter 12 may be a stirling converter. When converter 12 is operating, a small portion of the electricity generated by converter 12 is transferred through power line 16 to electromagnet 18.

Electromagnet 18 is mounted to a base plate 28 that aligns electromagnet 18 with heat pipe 20. Base plate 28 may be part of heat transfer structure 24 or a separate plate. Electromagnet may be mounted to base plate 28 by any means known in the art, including, but not limited to: machining, welding, or gluing. In one embodiment, base plate 28 is formed of metal to increase heat transfer from heat pipe 20. When converter 12 is in operation, electricity is fed through power line 16 to electromagnet 18, enabling electromagnet 18 to maintain heat pipe 20 in a position spaced from converter 12 (first position).

Heat pipe 20 has a first end 30 facing electromagnet 18 and a second end 32 facing converter 12. A wick 34 is positioned within interior 36 of heat pipe 20. A working fluid WF, present in the liquid and vapor phases, is also contained within heat pipe 20. In one embodiment, working fluid WF may be a coolant, including, but not limited to: water, ethanol, ammonia, mercury, potassium, and sodium. A heat transfer plate 38 is connected at second end 32 of heat pipe 20 and is formed of a material having good heat transfer properties. For example, heat transfer plate 38 may be formed of metal. When converter 12 is operational, heat pipe 20 is spaced from converter 12.

Heat pipe 20 is movable between a first position and a second position. When heat pipe 20 is in the first position, heat pipe 20 is spaced from converter 12. When heat pipe 20 is in the second position, heat pipe 20 is adjacent converter 12. Spring 22 is positioned between electromagnet 18 and first end 30 of heat pipe 20 and exerts a force against heat pipe 20 toward converter 12. Even though the force from spring 22 is acting against electromagnet 18 to push heat pipe 20 toward the second position, when converter 12 is operational, electromagnet 20 is powered on and is sufficient to overcome the force of spring 22. Although FIG. 1 is discussed as using an electromagnet and spring mechanism to maintain heat pipe 20 in the first position, any mechanism known in the art may be used, including, but not limited to: a mechanical switch or a permanent magnet proximate electromagnet that would reduce the amount of current needed to create force in the electromagnet.

First end 30 of heat pipe 20 is surrounded by heat transfer structure 24, which is a formed of a material having good heat transfer properties. An additional heat transfer material 40 may also be positioned between heat pipe 20 and heat transfer structure 24 in order to increase contact and to facilitate heat removal between heat pipe 20 and heat transfer structure 24. Heat transfer structure 24 may be formed of any material having good heat transfer properties. For example, heat transfer structure 24 may be formed of metal or metallic foam, including, but not limited to: aluminum, copper, steel, or superalloy. Heat transfer material 40 may be formed of any material that provides good contact to heat pipe 20, such as a solid structure or a metallic foam.

FIG. 2 is a schematic of power supply system 10 when converter 12 is not operational. When converter 12 is not generating electricity, electromagnet 18 does not receive any power and is shut off. Because electromagnet 18 is turned off, electromagnet 18 is no longer capable of holding heat pipe 20 in the first position. The force from spring 22 thus moves heat pipe 20 to the second position such that heat pipe 20 engages converter 12. Because converter 12 is formed of metal, when heat pipe 20 and heat transfer plate 38 are in contact with converter 12, the heat from converter 12 is easily transferred through heat transfer plate 38 to heat pipe 20. As the heat is transferred to heat pipe 20, circulation of working fluid WF within heat pipe 20 is initiated.

Heat pipe 20 uses evaporative cooling to transfer thermal energy from second end 32 of heat pipe 20 to first end 30 of heat pipe 20. As the contact time between heat transfer plate 38 and converter 12 elapses, working fluid WF at second end 32 of heat pipe 20 in contact with heat transfer plate 38 begins to boil and evaporate, increasing the vapor pressure within interior 36 of heat pipe 20. As working fluid WF evaporates, it absorbs thermal energy transferred from converter 12. Because first end 30 of heat pipe 20 is not in contact with converter 12, it has a lower temperature. The pressure difference between first end 30 of heat pipe 20 and second end 32 of heat pipe 20 moves the vapor toward first end 30 of heat pipe 20. At first end 30 of heat pipe 20, working fluid WF begins to condense back to the liquid phase and releases thermal energy. As working fluid WF is condensed, it is absorbed by wick 34 and flows back toward second end 32 of heat pipe 20 through the capillary action of wick 34.

The thermal energy released by working fluid WF at first end 30 of heat pipe 20 is transferred through heat pipe 20 to heat transfer material 40, which provides good contact between heat pipe 20 and heat transfer structure 24. Heat transfer structure 24 then releases the excess heat to the atmosphere in order to maintain the temperature of heat pipe 20 at a proper operating temperature. Base plate 28 may also assist in withdrawing heat from heat pipe 20 and releasing it into the atmosphere. The excess heat may also optionally be sent to a radiator for disposal.

FIG. 3 is a diagram of a method 100 of removing heat from power supply system 10. Heat pipe 20 is initially maintained in a first position spaced from converter 12 by electromagnet 18, Box 102. As depicted in Box 104, when converter 12 fails, power is disconnected between converter 12 and electromagnet 18, causing electromagnet 18 to shut off. The force from spring 22 is then sufficient to move heat pipe 20 from the first position to the second position adjacent converter 12, Box 106. When heat pipe 20 engages converter 12, heat is transferred from heat source 14 to converter 12 to heat pipe 20, Box 108. Lastly, as depicted in Box 110, the heat is withdrawn through heat pipe 20 into the atmosphere or to a radiator for disposal.

The power supply system generally includes a heat source, a converter, an electromagnet connected to the converter through a power line, a spring, a heat pipe, and a heat transfer structure. When the converter is operating, it draws heat from the heat source in order to produce electricity. A portion of the electricity is used to power the electromagnet, which maintains the heat pipe in a first position relative to the converter. When the heat pipe is in the first position, it is spaced from the converter. When the heat pipe is in a second position, it is adjacent the converter. The electromagnet is sufficient to overcome a force exerted on the heat pipe by a spring. The spring is pushing the heat pipe towards the converter.

Upon converter failure, the electromagnet is no longer powered on and the spring forces the heat pipe toward the converter. Heat is transferred from the converter to the heat pipe, which includes a wick and a working fluid. The working fluid and the wick make the heat pipe self-sustained and function to remove heat from the converter. The thermal energy within the heat pipe is then transferred to the heat transfer structure, which transfers thermal energy to either the atmosphere or a radiator. Because the power supply system has an alternate means of removing heat from the converter, it increases the safety of the system, reducing the likelihood of the iridium cladding overheating.

Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. 

1. A power supply system for converting heat to energy, the power supply system comprising: a heat source; a converter positioned adjacent the heat source for using heat from the heat source; a heat pipe having a first end and a second end and being capable of withdrawing heat from the converter; a heat transfer structure surrounding the heat pipe; and a mechanism for moving the heat pipe between a first position spaced from the converter and a second position adjacent to the converter as a function of electrical power generated by the converter.
 2. The power supply system of claim 1, wherein the heat pipe houses a wick and a working fluid.
 3. The power supply system of claim 1, wherein the heat transfer structure is formed of a metal.
 4. The power supply system of claim 1, wherein the mechanism comprises: an electromagnet connected to the heat pipe for maintaining the heat pipe in the first position; and a spring positioned at the first end of the heat pipe for moving the heat pipe toward the second position.
 5. The power supply system of claim 4, and further comprising a base plate for mounting the electromagnet.
 6. The power supply system of claim 1, and further comprising a metallic plate attached to the second end of the heat pipe for facilitating heat transfer from the converter to the heat pipe when the heat pipe is in the second position.
 7. The power supply system of claim 1, and further comprising a heat transfer material positioned between the heat pipe and the heat transfer structure.
 8. A system for drawing heat from a heat source, the system comprising: a converter positioned adjacent the heat source; a heat pipe capable of removing heat from the converter; a metallic heat transfer structure surrounding the heat pipe; and a mechanism for sliding the heat pipe between a first position and a second position as a function of electrical power generated by the converter.
 9. The system of claim 9, wherein the mechanism comprises: an electromagnet connected to the heat pipe for maintaining the heat pipe in the first position; and a spring positioned within the electromagnet and in contact with the heat pipe.
 10. The system of claim 9, wherein a power line supplies electricity from the converter to the electromagnet.
 11. The system of claim 8, wherein the heat pipe is spaced from the converter when the heat pipe is in the first position, and wherein the heat pipe is adjacent the converter when the heat pipe is in the second position.
 12. The system of claim 8, and further comprising a plate attached to the heat pipe for facilitating heat transfer from the converter to the heat pipe when the heat pipe is in the second position.
 13. The system of claim 8, and further comprising a heat transfer material positioned between the heat pipe and the metallic heat transfer structure.
 14. The system of claim 13, wherein the heat transfer material is metallic foam.
 15. A method of removing heat from a converter, the method comprising: establishing a power connection between the converter and an electromagnet, wherein the electromagnet is connected to a heat pipe; maintaining the heat pipe in a first position spaced from the converter; moving the heat pipe from the first position to a second position relative to the converter in response to the power connection between the converter and the electromagnet being disconnected; withdrawing heat from the converter to the heat pipe; and transferring the heat from the heat pipe.
 16. The method of claim 15, wherein moving the heat pipe from the first position to the second position comprises engaging the heat pipe with the converter.
 17. The method of claim 15, wherein maintaining the heat pipe in the first position comprises using the electromagnet.
 18. The method of claim 15, wherein withdrawing heat from the converter to the heat pipe comprises using a metallic heat transfer plate positioned between the converter and the heat pipe.
 19. The method of claim 15, wherein disconnecting the power connection between the converter and the electromagnet comprises releasing the heat pipe from the first position.
 20. The method of claim 15, wherein transferring the heat from the heat pipe comprises using a heat transfer structure. 